M. Heydari-Malayeri - Paris Observatory

A → photometric system defined by reference to
→ monochromatic magnitudes in such a way that, when monochromatic
→ fluxfν is measured in
ergs sec-1 cm-2 Hz-1,
the magnitude will be:
AB = -2.5 logfν - 48.60.
The constant is set so that AB is equal to the V magnitude
for a source with a flat → spectral energy distribution.
The → zero point is defined by the flux of the star
→ Vega at 5546 Å.
In this system, an object with constant flux per unit frequency interval has zero color.

A system of three stars, the → close binary
Alpha Centauri A (→ spectral type G2 V)
and Alpha Centauri B (K1 V), and a small and faint → red dwarf,
Alpha Centauri C (M6 Ve), better known as → Proxima Centauri.
To the unaided eye,
the two main components (AB) appear as a single object with an
→ apparent visual magnitude
of -0.27, forming the brightest star in the southern
constellation → Centaurus
and the third brightest star in the night sky, after
→ Sirius and → Canopus.
The individual visual magnitudes of the components A, B, and Proxima are +0.01, +1.33,
and +11.05, respectively. The masses of A and B are 1.100 and 0.907 Msun, respectively.
Their → effective temperatures are (A) 5,790 K and (B)
5,260 K; their luminosities (A) 1.519 Lsun and (B) 0.500 Lsun.
The binary members are separated in average by only 23
→ astronomical units.
They revolve around a common center of mass with a period of about 80 years.
Both have a distance of 4.37 → light-years.
Proxima Centauri, lying about 15,000 AU apart from AB, is
→ gravitationally bound to them.
It has a mass of 0.1 Msun, a radius of 0.1 Rsun, a luminosity
of about 0.001 Lsun, and an → effective temperature
of ~ 3,000 K.

A → numeral system that has 2 as its base and uses
only two digits, 0 and 1. The positional value of each digit in a binary number
is twice the place value of the digit of
its right side. Each binary digit is known as a bit. The decimal numbers from 0 to 10
are thus in binary 0, 1, 10, 11, 100, 101, 110, 111, 1000, 1001, and 1010.
And, for example, the binary number 111012 represents the decimal number
(1 × 24) + (1 × 23)
+ (1 × 22) + (0 × 21) + (1 × 20),
or 29. In electronics, binary numbers are the flow of information in the form
of zeros and ones used by computers. Computers use it to manipulate and store all of
their data including numbers, words, videos, graphics, and music.

A → binary system in which the distance separating
the stars is comparable to their size. Most close binaries are spectroscopic binaries
(→ spectroscopic binary) and/or eclipsing binaries
(→ eclipsing binary). In most of them
→ mass transfer occurs at some stage,
an event which profoundly affects the → stellar evolution
of the components.
The evolution of close binaries depends on the
→ initial masses of the two stars and
their → separation.
When the more massive star evolves into a
→ red giant first, material will spill through the inner
point onto its companion, thereby affecting its companion's evolution.
Mass transfer can also alter the separation and
→ orbital period of the binary star.

A binary star system which is composed of a collapsed object
(→ degenerate dwarf, → neutron star,
or → black hole) in orbit with a low-mass (≤ 0.5 Msol)
secondary star, wherein the collapsed star → accretes
matter from its → companion.
These two objects form a binary system of overall dimensions
106 km with an orbital period of only hours or less.
See also:
→ X-ray binary.